This invention relates to a vacuum chuck and, more particularly, to a vacuum chuck for supporting and rigidly clamping a workpiece during cutting of the workpiece by the cutting tool of a machine tool. The present vacuum chuck is particularly useful in connection with a machine tool such as a router having a rotating router bit which traverses across and cuts through a large sheet metal workpiece and forms the workpiece into smaller individual piece parts of various shapes.
A vacuum chuck conventionally includes a lower base having a vacuum plenum which is covered by a horizontal metal top plate formed with a series of vertically extending holes. When vacuum is applied to the plenum, the workpiece is clamped to the top plate by virtue of the vacuum acting through the holes and against the lower side of the workpiece.
Difficulty is encountered in using a vacuum chuck in operations such as a routing operation where the tool cuts completely through the workpiece. In such situations, steps must be taken to prevent the lower end of the tool from cutting into and ruining the metal top plate of the chuck. One solution to this problem is to provide so-called spoiler plates for spacing the workpiece above the top plate.
A spoiler plate conventionally comprises a sheet of soft wood or particle board having a shape similar to the shape of the piece part to be cut and having peripheral dimensions the same as or just slightly larger than the piece part. A groove having the same general shape as the piece part and having peripheral dimensions slightly less than the piece part is formed in the upper side of the spoiler plate and serves as as retainer for a gasket made of closed cell foam and adapted to project a short distance above the spoiler plate. In addition, a cavity or plenum having the same general shape as the piece part is formed in the upper side of the spoiler plate within the confines of the gasket. A vertical hole is formed through the spoiler plate in order to establish communication between the plenum of the spoiler plate and the vacuum holes in the top plate of the vacuum chuck.
The spoiler plate is placed on the top plate of the vacuum chuck in underlying relation with that area of the workpiece from which the particular piece part is to be cut. When vacuum is applied to the chuck, it acts within the plenum of the spoiler plate to draw the workpiece downwardly against the spoiler plate. As the workpiece is drawn downwardly, the gasket compresses so as to seal around the plenum in the spoiler plate and thereby cause the workpiece to be clamped tightly to the spoiler plate. During the cutting operation, the tool cuts through the workpiece and into the upper surface of the soft spoiler plate but does not reach to the metal top plate of the chuck since the spoiler plate holds the workpiece in vertically spaced relation with the top plate.
When the piece parts of many different sizes and shapes are cut from workpieces, the use of spoiler plates is expensive, time-consuming and cumbersome. A customized spoiler plate must be made for each piece part of different size or shape and must be placed on the top plate of the vacuum chuck in the specific area of the piece part. Each time the machine tool is changed over to cut different piece parts, the previously used spoiler plates must be replaced with different spoiler plates. Significant floor space is required to store all of the customized spoiler plates necessary for a multitude of piece parts of various sizes and shapes.